We propose to study patients with antibiotic-induced colitis or diarrhea. We will define whether Clostridium difficile, Clostridium perfringens type C, or Staphylococcus aureus are responsible for these conditions. We will attempt to develop better means of diagnosing these conditions employing selective cultural media, and/or newer immunologic test. We will determine the value of vancomycin in treating these conditions, as well as the frequency of and reasons for relapses and recurrences. We will determine the frequency of spread of the responsible organisms to the patient's environment and will relate this to development of the disease by others contacting that environment.